1. Field of the Invention
The present invention relates to a bias circuit and a power amplifier with a power mode selection function, which may be applied to a wireless communications system.
2. Description of the Related Art
In general, a wireless communications scheme is a digital modulation/demodulation scheme, and an appropriate scheme has been employed in consideration of improvements in frequency utilization efficiency. For instance, a quadrature phase shift keying (QPSK) scheme is employed for a mobile telephone using a code division multiple access (CDMA) scheme, and a digital modulation scheme of an orthogonal frequency division multiplexing (OFDM) scheme is employed in a wireless LAN conforming to the institute of electrical and electronics engineers (IEEE) 802.11 standard.
A wireless communications system in which such wireless communications schemes are employed includes a power amplifier for amplifying the power of a transmission signal.
Here, in a system requiring linear amplification, a power amplifier which has a linear characteristic for amplifying the transmission signal without distortion is required. Here, the linear characteristic indicates that the power of an output signal is amplified at a constant rate while the phase thereof is not changed, even when the power of an input signal may vary.
In addition, in a wireless communications apparatus, in order to increase a usage time thereof within a limited battery capacity, technology for significantly reducing power consumption in components to thereby increase efficiency is needed.
In a typical power amplifier, a bias voltage generated by a bias circuit is supplied to a power amplifying element through a resistor. In this case, the bias circuit may include a temperature compensation function.
However, since the above-mentioned typical power amplifier provides a constant bias voltage after being designed, a high power mode or a low power mode may not be able to be selected as necessary. Therefore, it may be difficult to take an appropriate action for transmission power required for device characteristics or the communications environment of a wireless communications system to which such an amplifier is applied.
The Related Art Document relates to a power amplifier, but does not disclose a technical feature allowing for a high power mode or a low power mode to be selected in order to decrease power consumption.